Signal on formula evaluation

TODO.md

@@ -30,4 +30,6 @@
   - [X] From now on, the corpus will be loaded from database instead of assets
 - [X] Create method List<FormulaElement> Corpus.withDependencies(Formula formula). It will return the formula, the units of the formula, and all the units from the corpus with the same base unit.
 - [X] Add a Share button to the formula list. It will export the array string literal of the formula with the units from Corpus.withDependencies().
-- [ ] Replace flutter-markdown with flutter-markdown-plus
+- [X] Replace flutter-markdown with flutter-markdown-plus
+- [X] Heron's formula: investigate why a=3, b=40, c=5 yields NaN. Root cause: input values don't form a valid triangle (violate triangle inequality: 3+5=8 is not > 40). Added documentation note to the formula description.
+- [ ] Investigate starup time when there is no previous database and corpus is loaded from assets.

assets/formulas/formulas.d4rt

@@ -1036,7 +1036,9 @@ Where:
 - $a, b, c$: Side lengths (meters)
 - $s$: Semi-perimeter $= \frac{a+b+c}{2}$
 
-This formula is useful when height is unknown.''',
+This formula is useful when height is unknown.
+
+**Note:** The side lengths must satisfy the triangle inequality: the sum of any two sides must be greater than the third side (a+b>c, a+c>b, b+c>a). If this condition is not met, the formula returns NaN.''',
     "input": [
       {"name": "a", "unit": "meter"},  // Side 1
       {"name": "b", "unit": "meter"},  // Side 2
@@ -1044,6 +1046,9 @@ This formula is useful when height is unknown.''',
     ],
     "output": {"name": "A", "unit": "square meter"},  // Area
     "d4rtCode": """
+       if( a + b < c || a+c < b || b+c < a ){
+          signal( "There is not a valid triangle with those longitudes" );  
+       }
        var s = (a + b + c) / 2;
        A = sqrt(s * (s - a) * (s - b) * (s - c));
     """,

lib/formula_evaluator.dart

@@ -162,6 +162,10 @@ class FormulaEvaluator {
       import "package:d4rt_formulas.dart";
   """;
 
+  static final String signal = """
+      void signal( String msg ) => throw Exception(msg);       
+  """;
+
   static const reservedVariableNames = { "variableValues", "indexOf", "variableAllowedValues"} ;
 
   String _buildCompleteSource(Formula formula, Map<String, dynamic> inputValues) {
@@ -169,7 +173,7 @@ class FormulaEvaluator {
 
     buffer.writeln("""
       $d4rtImports
-      
+      $signal
       main()
       {
       """